Ionizing radiation apparatus and method for distinguishing between materials in a mixture

ABSTRACT

Materials in a mixture are distinguished for identification or sorting purposes by subjecting them to ionizing radiation at least two different energy levels and determining the effects, other than solely edge effects, of the materials on the two levels of radiation.

l United States Patent [151 3,655,964 Slight Apr. 11, 1972 [54] IONIZING RADIATION APPARATUS [56] References Cited AND METHOD FOR DISTINGUISHING UN BETWEEN MATERIALS IN A MIXTURE STATES TENTS Inventor: David Laurie g North Mains 2,884,535 4/1959 Stsflft, Jr. ..250/83.3 D ormiston East Lothian Scotland 2,947,871 8/1960 Fnedman ..250/83.3 D 3,082,323 3/1963 Chope et al ..250/43.5 D [22] FIled: May 6, 1968 3,270,204 3/1966 Rhodes ;....250/43.5 D 2 Appl 72 92 3,452,192 6/1969 Hanken .l ..250/43.5 D

Related US. Application Da Primary Examiner-Archie R. Borchelt [63] Continuation-impart of Ser. No. 433,705, Feb. l8, and Hmds l 965, abandoned.

, [57] ABSTRACT '5" "250/415 I gag 25 2 Materials in a mixture are distinguished for identification or [58] i 5 83 3 sorting purposes by subjecting them to ionizing radiation at least two different energy levels and determining the effects, I other than solely edge effects, of the materials on the two levels of radiation.

8 Claims, 4 Drawing Figures POLARISED *fi P RELAY 70" ,9 9 X-RAY X-RAY r-swaw D.C.AMPLIHER T T/51 p DLAMPUFIER POWER BACK CONTROL SUPPLY [M'EGRATQR lNTEGRATOR TIMER INTEGRATOR A a PULSEAMPLIFIER AND 2 CHANNEL PULSE HEIGHT ANALYSER ION IZING RADIATION APPARATUS AND METHOD FOR DISTINGUISHING BETWEEN MATERIALS IN A MIXTURE CROSS-REFERENCE TO RELATED APPLICATIONS The present application is a continuation-in-part of application Ser. No. 433,705, filed on Feb. 18, 1965, and now abandoned.

This invention relates to arrangements for and methods of analysing and/or sorting materials, especially those arrangements or methods suitable for analysing and/or sorting a mixture of a plurality of materials. The invention is based upon the fact that most, if not all, elements have different radiation absorption (and presumably scattering) coefficients over a range of photon levels. (See, for example, Cameron, J. F. The Use of Radioactive Isotopes in Non-destructive Testing. Progress in non-destructive testing. Vol. 2. p.91 1959)).

Proposals have been made heretofore for apparatus to determine the composition and thickness of a layer of material by use of X-rays and these proposals have in fact been particularly suited to control of composition of material in production in continuous flow. The proposals have, however, relied upon use of radiation radiating at two energy levels, one just above and the other just below the absorption edge of the element being monitored; it is another limitation that the proposals have been capable of monitoring only one element. Furthermore, use of the absorption edge effect places a further limitation on thickness of the layer being monitored and only thin sheets of materials, such as metal or plastics, films or foils, or paint films on standard bases are capable of being monitored by this known apparatus.

There is a demand for apparatus that is capable of providing rapid identification or analysis and such like of mixtures of discrete lumps of materials of differing sizes and compositions and it is an object of the present invention to provide such an apparatus.

It is a particular feature of use of the present invention, therefore, that discrimination may be effected between different materials in spite of their being of different thicknesses and masses.

An arrangement for analysing and/or sorting a mixture of two or more materials in accordance with the invention comprises means for subjecting said materials to radiant energy at or including a plurality of energy levels, means responsive to the effects of said materials upon said radiation at two or more different energy levels and means under the influence or control of said responsive means, for indicating the composition of at least one of said materials and/or for identifying and/or discriminating between and/or sorting at least some of the said different materials in dependence upon the response of said responsive means at said two or more energy levels. The radiant energy may be substantially continuous over the whole range of the said energy levels and response at the required different levels may be derived from analysis of the response of said responsive means to said continuous range of radiant energy. Alternatively the radiant energy may be derived from sources of different gamma-radiations, the number of different radiations being at least equal to the number of different materials to be analyzed and/or separated out; it may be possible to obtain the requisite number of different radiations from a single source of gamma-radiation, emitting in a plurality of modes.

Signals from the responsive means may be passed to means for subjecting them to analysis at each of the said plurality of levels, said energy level analysis means being adapted to pass their signals of analysis to means for submitting the same to comparison with standard signals, whereby one or more of the materials may be identified and/or sorted or it, or their, composition(s) indicated.

In accordance with a feature of the invention an arrangement for monitoring and/or determining the composition of a mixture of materials in substantially homogeneous form, such as for process control, comprises means for subjecting said mixture in motion, or in turn, to radiant energy at or including a plurality of energy levels, means responsive to the effects of said materials upon said radiation at two or more different levels, the number of levels depending upon the number of discrete materials in the mixture the composition of which it is desired to monitor and/or determine, means adapted to compare said effects on said responsive means at the said two or more energy levels with similar effects caused by materials of standard composition and means sensitive to said response for indicating coincidence with and/or departures from said standard.

In accordance with yet another feature of the invention an arrangement for sorting one or more materials from a mixture with one or more other materials having different response(s) to the incidence of ionising radiation, comprises means for providing a beam of radiant energy at or including a plurality of energy levels, means for causing said mixture to pass through said beam, means for detecting the response of said different materials to the effects of incidence of two or more of said levels of radiant energy, means for comparing the response of their effects at the two different levels or more with standard responses or their efiects at the respective levels, diverting means in the path of said materials after passing through said beam being provided under the control of signals from said comparison means for said responsive means, for diverting the or those material(s) for which there is coincidence with (or departure from) standard response for that particular material selected. The said diverting means may comprise electro-mechanically controlled keys adapted to be operated in accordance with signals from said comparison means to divert or let pass certain of the different materials depending upon the information supplied by the comparison means concerning said materials. If two or more materials are to be separated from each other and from one or more other materials, a plurality of sets of said diverting means may be arranged in series, the different sets being adapted to operate in accordance with the signals derived from said comparison means for different materials in said mixture, whereby said different materials may be segregated from each other by operation of one or other of said diverting means.

In accordance with another feature of the invention a method of analysing and/or sorting a mixture of two or more materials having different response to the incidence of ionizing radiation from each other, comprises subjecting said materials to radiant energy at or including a plurality of energy levels, assessing the effects of said materials upon said radiation at two or more different energy levels, said two or more different energy levels being chosen to give different responses of at least one or some of said materials, and by comparison of said assessment with known responses of each said material or selected material, identifying the composition of the said one or more materials or causing said one or more materials to be separated from the remainder.

Since the invention relies, in effect, upon comparison of absorption coefficients at the different energy levels, it is clear that absorption edges are to be avoided and that energy levels must be chosen so as both to lie to one side or the other of any prominent edge of the materials under examination. The spectrum of energy levels used for the present proposals must not span the absorption edge of any of the elements concerned.

The energy links should be chosen such that the bandwidth of the detection means is sufficiently far removed from any absorption edge as not to impair the required accuracy of the final analysis. Generally speaking the greater the accuracy required, the further should the energy limits be from the absorption edge.

In order that the invention may be clearly understood, some embodiments thereof in different forms will now be described by way of examples with reference to the accompanying drawings.

In FIG. 1 of the drawings is sown in diagrammatic form an apparatus for separating out one material from a mixture with another material, such as, for instance, flint form a chalk/flint mixture so as to leave a substantially flint-free chalk.

FIG. 2 illustrates, also diagrammatically, a suitable arrangement of X-ray beam generator, conveyor and detection system for use in recording, or monitoring, the flow of a mixture of solids, while FIG. 3 is an indication of an apparatus that would be suitable for use with a flow of powder or slurry, wherein the composition of the powder or slurry may be monitored or recorded.

FIG. 4 is a block diagram of apparatus suitable for analysing or sorting a plurality of more than three materials from a mixture with one or more further materials.

Referring now to FIG. 1, a mixture that may contain two or more different materials M and M is fed by a conveyor 1 so as to fall across an X-ray beam which is emitted by an X-ray tube in housing 2 through a collimator 3. The collimator provides a thin beam that is broad enough to span the width of the conveyor. Various forms of beam generator are described in British Pat. No. 984,232 and it is possible that the generator could be pulsative. It is necessary to ensure that the radiant energy emanating from the collimator covers at least two energy levels, the absorption of which by one material to be sorted out of the mixture is such that the ratio of the response to the detectors at the two energy levels in the presence of the one material is appreciably different from the ratio in the presence of the other or any of the other materials.

A number of scintillators 4 are arranged at discrete intervals across the width of the beam and these are normally exposed to the beam; each scintillator is associated with a scintillation counter 5 in known manner but, if desired, these scintillators and counters may be replaced by proportional counters and as will be seen later, the beam may be controlled to prevent overloading of the scintillators.

The signals from each counter are fed to a pulse analyser 6 which includes a pulse amplifier and the pulses are divided into two pulse height channels A, B. Each channel feeds an integrator 7A, 78, such as a diode pump integrator, and a DC. amplifier 8A, 8B. Integration is arranged to be effective over timed periods, these being triggered by a start/stop timer 9 and the outputs are fed to a polarized relay 10, or the equivalent.

A control signal is taken from one of the channels A, B, from the pulse height analyser through a feed back control integrator 11 to the power supply 14 to control the current in the X-ray tube in such a way that the tube current is reduced for a high count rate and increased for a low count rate this means in fact that the tube current may be reduced to zero, for example, for air and that the tube current will be low for a thin piece of material and increased for a thick piece of material and in this way the detection system is not overloaded.

The relay may be arranged to operate about a balance point such that the presence in the radiant beam of a piece of the material to be sorted out will cause the relay to operated to energise an electromechanical, or a pneumatic actuator 11, through an amplifier 12 if required, so as to prevent the appropriate finger 13 of a separator from deflecting when a piece of the particular material strikes it. The result of that operation of the relay will therefore be to divert the one material from the general stream of falling materials, other materials simply deflecting to allow passage of the material(s). Reference may be had to the specification and drawings accompanying US. Pat. No. 3,209,910 for further details of conveyor and diverting mechanisms.

Thus if the material fed by the conveyor 1 is a mixture of chalk with flints, the separating apparatus can be made to operate so that flint is the material which causes locking of the appropriate separating fingers so that the flint is taken out of the stream of chalk.

It will be understood that if desired a plurality of detector banks and/or of sorting layers may be provided.

The apparatus shown in FIG. 2 would be associated with a pulse height analyzer in this way and integrators similar to those shown in the arrangement of FIG. 1, but the signals from the integrators may be arranged to feed a recorder or some similar device so that the composition of the flow of materials may be monitored or recorded. In FIG. 3, the conveyor for solid materials in FIG. 2 is replaced by a rectangular section duct through which a liquid slurry or airborne powder may be directed and the composition of the powder or slurry can be recorded, monitored or analysed as will be evident. In the arrangements of both FIG. 2 and FIG. 3 the signal derived from the detectors will be backed off to a certain extent because of the presence of the conveyor belt or the ducting.

It will be understood that each scintillation counter channel will be associated with its own anaylser circuit and that, therefore, each finger of the separator in the arrangement of FIG. 1 will be under the control of its appropriate counter through the particular circuits associated therewith. It will also be evident that there is no limit to the number of counters arranged across the width of the conveyor; and also that the width of the conveyor need not be restricted, though some difficulty may be encountered with the spread of a single source of X-rays and it may be necessary to provide more than one source, though such sources will essentially be similar. In addition the smaller the scintillator and counter assembly, the more sensitive may the arrangement be made to size of material but the complication of the system will increase as the number of counters is increased, since each will need to have its own analysing system for control of separators or for monitoring etc.

Referring now to FIG. 4, a system is illustrated that can be used for identifying and/or analysing a multiplicity of materials. Here again it will be understood that the parts of the apparatus illustrated are only typical and that provision will need to be made for similar parts for each counter involved. A plurality of gamma-ray sources 20 are arranged to direct the beam of gamma rays on to counters 21 which may be of the scintillation or proportional type. As discussed above, the X- ray source described hereinbefore can be replaced by a number of gamma-ray sources or by a single source of gammaradiation emitting in a plurality of modes. There are many gamma-ray sources on the market and all one needs to do is to consult a list from a supplier and choose the sources best suited to the particular materials to be examined. It is clear that sources whose half-lives are very short are unacceptable but having decided upon the particular sources and hearing this limitation in mind, one need only to determine in a wellknown manner the appropriate quantity of each source necessary to provide a signal of appropriate strength. The output pulses from each counter are fed to a linear amplifier 22 and the signal is divided into two sets of different pulse heights which are fed to analysers 23A and 233; each set is subsequently integrated in diode pump integrators 24A and 248 at least until a statistical significance is established and the resulting signals are fed into a simple form of analogue computer 25. In the computer it may be arranged, for instance, that the times taken to charge standard capacitors to preset voltages may be established and a signal proportional to the ratio of these times for the two channels is fed to a store 26 of information on each type of material or object or the signal may be compared with standard ratios and the material or object thereby identified or analysed.

Thus each counter operates to provide packets of information concerning each object or piece of material that passes between it and the radiation sources; and these packets of information are analysed and eventually provide the means of identifying the particular object or pieces of material. If analyses of objects or the pieces of the material are required to be obtained by such a means, it is almost essential that they should be homogeneous, but such requirement will depend to a large extent upon the accuracy with which the apparatus is required to work. It will be seen that if the material consists of a plurality of constituent parts, it is necessary that the number of levels of radiant energy, and therefore the number of channels of pulse heights, should be at least equal to the number of said constituents. The computer may then function, if necessary, to provide an indication of the quantity of each constituent present in a flow of material, or on a continuous basis. Alternatively, the apparatus may be used to search for particular materials or constituents and to signal their presence; the apparatus may even measure the quantity or quantities present.

Another application of apparatus similar to that illustrated in FIG. 4, is to separate out a mixture of more than two components. Thus sorting apparatus may be similar to that shown in FIG. 1, but the number of separators may be increased to equal the number of components to be taken out of a mixture. The additional separators, which may each comprise a plurality of controlled fingers in the same way as a separator in FIG. 1, will be arranged in series, one vertically below the next. The computer will then assess the pulses from each counter as pieces of the mixture pass through the beam and, by means of gating techniques, in well-known manner, will provide discriminating signals which will cause locking of the appropriate finger(s) of the particular separator for diverting the material subjected to the particular scrutiny. The finger thus locked will ensure that the particular piece of material is directed from the flow at the correct stage. I

It will be observed teat, using the above techniques, mixtures may be fed over a broad front, or a narrow one, as

desired. Of course, it only a small through-put is required, it

may be arranged that each piece of the mixture passes through the same part ofthe radiation and only one counter, and associated gear, need be provided. Of course if the source is of variable amplitude the apparatus can be arranged to be set atthe most advantageous levels for any of a whole variety of constituents. By providing for working over a wide enough spectrum of energy levels, any particular material may be scrutinised for the presence of selected constituents.

It would appear that an advantageous range of energy levels in which apparatus or a method in accordance with the invention is approximately 10 100 Kev.

It is to be understood that the scope of the present invention is not confined to those embodiments in which it is arranged that the materials of the mixture shall traverse the radiation beam completely. Indeed, where the detection system is arranged to detect effects due to scatter of the radiation falling upon the materials, it may be necessary, and even desirable, that the materials shall pass through only a part of the beam thickness, that is to say, for example, merely into the path of the radiant energy.

To understand the operation of the apparatus or the method of the invention, it is necessary to take the basic equation for X-ray (or other radiation) absorption by a material. This is 7 3 #5 where I intensity emergent from the material,

1 intensity incident upon the material absorption coefficient of material and x thickness of the material and it is known that u for a particular material is not constant but varies with the energy level of the radiation.

Therefore, where a substantially constant composition of mixture is passed through the apparatus, measurement of the emergent intensity of the radiation at two energy levels by means of the detector, amplifier and analyser results in the establishment of two signals to be passed to the computer:- i.e.

' proportional to The computer may solve equation 1 for x using for the normal mixture at that energy level, and that value of x is then substituted in equation 2 which is solved for m. If this value of ,U-z is the same as the absorption coefficient of the normal mixture at that energy level, the mixture under test is normal, and if not, the mixture is other than normal. Thus the output of the computer may be used either to that the computer calculation is independent of the flow thickness.

Where, in another application of the invention, it is desired to identify, in respect of its composition and irrespective of its thickness, each object in a mixture of objects composed of, say, a number of materials whose absorption coefiicient pt is known for particular levels, the same measurements are taken as for the fist application and the computer then solves the simultaneous equations in the same way except that if the final comparison of p does not tally, the comput.-r recalculates using a different value of a, corresponding to a different material. When equality of 14. is achieved the material is identified and its thickness has also been calculated, and this information can be used, for example in keeping records, in quality control or in separation.

Where in a third application of the invention, it is desired to find the proportion of each of two or more known materials in a mixture, measurements are made at two or more energy levels, the number of energy levels being equal to the number of materials. The simultaneous equations resulting are solved by the computer and the proportions of the various materials provided.

It will be appreciated that the response of the various parts of the circuits described may readily be made sufficiently rapid that all of the conditions described above for functioning of the apparatus may be satisfied during the time taken for a piece of material of smallest effective size to fall under gravity through a beam of comparable thickness. ln consequence apparatus in accordance with the invention may be made to be very versatile in application.

What is claimed is:

l. A method of identifying pieces of at least one material in a continuous flow of a mixture comprised of discrete pieces of at least two different materials, said method comprising establishing as a standard the ratio of the effects caused by at least one of the materials on ionizing radiation radiating at each of at least two levels, said levels being substantially different from and to one side only of at least a prominent absorption edge of each of the materials in said mixture, passing the flow of discrete pieces of said at least two different materials through radiation at least at said two levels, assessing the ratio of the eflect of each piece of material passing through the radiation on said at least two levels of radiation, and comparing the assessments with the standard effect, and thereby. identifying at least the pieces of said at least one material.

2. A method as claimed in claim 1 wherein said radiation comprises gamma-radiation and wherein said at least two levels of radiation are obtained by using different sources of gamma-radiation.

3. A method as claimed in claim 1 wherein said mixture includes first and second materials, said method further comprising sorting out pieces of said first material from pieces of .said second material using deflecting means responsive to a control signal produced in accordance with the results of said assessing step for deflecting pieces of said second material from said continuous flow.

4. A method as claimed in claim 1 wherein said mixture is radiated with a beam of ionizing radiation and said assessing step includes using a plurality of scintillators, arranged at discrete intervals across the width of said beam, and a plurality of counters associated with said scintillators, to produce output signals in accordance with the effect of said mixture on said radiation.

5. A method as claimed in claim 4 further comprising feeding back said output signals to control the radiation from said sources to prevent overloading of said scintillators.

6. A method as claimed in claim 5 wherein said output signals are fed back through an integrator to a power supply for said sources.

7. A method as claimed in claim 1 wherein said mixture includes first and second materials, said method further comprising sorting out pieces of said one material from pieces of said second material using deflecting means responsive to a record the abnormality or to correct it. It should be noted control signal derived from said output signals.

7 8 8. A method as claimed in claim 7 further comprising applysecond signal through a second integrator and second DC aming said output signals to a pulse amplifier and two channel p ifie i 8 Second np of Said Polarized r y, Said lay pulse height analyzer to produce first and second signals, ap- P ing aid Control signal. plying said first signal through a first integrator and a first DC i =0 1 amplifier to one input of a polarized relay and applying said 5 

1. A method of identifying pieces of at least one material in a continuous flow of a mixture comprised of discrete pieces of at least two different materials, said method comprising establishing as a standard The ratio of the effects caused by at least one of the materials on ionizing radiation radiating at each of at least two levels, said levels being substantially different from and to one side only of at least a prominent absorption edge of each of the materials in said mixture, passing the flow of discrete pieces of said at least two different materials through radiation at least at said two levels, assessing the ratio of the effect of each piece of material passing through the radiation on said at least two levels of radiation, and comparing the assessments with the standard effect, and thereby identifying at least the pieces of said at least one material.
 2. A method as claimed in claim 1 wherein said radiation comprises gamma-radiation and wherein said at least two levels of radiation are obtained by using different sources of gamma-radiation.
 3. A method as claimed in claim 1 wherein said mixture includes first and second materials, said method further comprising sorting out pieces of said first material from pieces of said second material using deflecting means responsive to a control signal produced in accordance with the results of said assessing step for deflecting pieces of said second material from said continuous flow.
 4. A method as claimed in claim 1 wherein said mixture is radiated with a beam of ionizing radiation and said assessing step includes using a plurality of scintillators, arranged at discrete intervals across the width of said beam, and a plurality of counters associated with said scintillators, to produce output signals in accordance with the effect of said mixture on said radiation.
 5. A method as claimed in claim 4 further comprising feeding back said output signals to control the radiation from said sources to prevent overloading of said scintillators.
 6. A method as claimed in claim 5 wherein said output signals are fed back through an integrator to a power supply for said sources.
 7. A method as claimed in claim 1 wherein said mixture includes first and second materials, said method further comprising sorting out pieces of said one material from pieces of said second material using deflecting means responsive to a control signal derived from said output signals.
 8. A method as claimed in claim 7 further comprising applying said output signals to a pulse amplifier and two channel pulse height analyzer to produce first and second signals, applying said first signal through a first integrator and a first DC amplifier to one input of a polarized relay and applying said second signal through a second integrator and second DC amplifier to a second input of said polarized relay, said relay producing said control signal. 